1. Field of the Invention
The present invention relates to a measurement system using a tracking-type laser interferometer and a return method of the measurement system. In particular, the present invention relates to a measurement system using a tracking-type laser interferometer and a return method of the measurement system which, even when a laser light is interrupted and tracking is no longer possible, is capable of automatically resetting the tracking and restarting measurement.
2. Description of Related Art
Japanese Patent No. 2,603,429, U.S. Pat. No. 6,147,748, and Japanese Patent No. 4,776,454 describe examples of a tracking-type laser interferometer which, while tracking a displacement body, measures displacement and a position of the displacement body with a high degree of accuracy. These tracking-type laser interferometers are used when calibrating a measuring device or a machine tool, and a retroreflector (recursive reflector) is mounted, as a measured body, to a device to be calibrated.
FIG. 1 illustrates an exemplary configuration of a tracking-type laser interferometer. A tracking-type laser interferometer 40 is configured by a retroreflector 42 fixated to a displacement body (object to be measured) 22; an optical measurement device 70; a biaxial rotation device 80; and a control device (hereafter referred to as a rotation mechanism control device) 82 for the biaxial rotation device 80. The biaxial rotation device 80 includes a biaxial rotation mechanism having mutually orthogonal axes, and has an angle detector (not shown in the drawings) on each axis. Laser light 74 is guided from a laser light source 60 to the optical measurement device 70 using an optical fiber 62. The optical measurement device 70 is configured by a laser interferometer (length measurement meter; hereafter, referred to simply as a laser interferometer) 72 measuring a distance L from an origin point O to the retroreflector 42; and an optical device for tracking 76 used in tracking control of the retroreflector 42.
The retroreflector 42 is an optical element in which the optical axes of incident light and reflected light are parallel, and the optical axes of the incident light and reflected light are point symmetrical with respect to the center of the retroreflector 42. Therefore, when the incident light strikes a position away from the reflection center of the retroreflector 42, the reflected light is returned to a position offset with respect to the incident light. The optical device for tracking 76 includes an optical position detector 78 monitoring an amount of offset between the incident light and the reflected light. The optical device for tracking 76 detects the amount of offset and transmits it to the rotation mechanism control device 82.
Using a signal (amount of offset between the incident light and the reflected light) sent from the optical device for tracking 76 and an angle signal output from the angle detectors on each of the rotation axes of the biaxial rotation mechanism, the rotation mechanism control device 82 controls the biaxial rotation device 80 so as to bring the amount of offset within a predetermined range.
The laser interferometer 72 attached to the biaxial rotation device 80, which is configured by a biaxial rotation mechanism (biaxial rotator) having mutually orthogonal axes, causes interference in light returning from the retroreflector 42; includes a detector detecting by phase a change in intensity of the interference light; and treats a rotation center (intersection point) of the biaxial rotation mechanism as the origin point O, and the distance L from the origin point O to the retroreflector 42 is measured via the rotation mechanism control device 82.
With this configuration, in a measurement of the tracking-type (tracking) laser interferometer 40, an angle signal of the biaxial rotation device 80 and distance data (L) observed by the laser interferometer 72 are obtained as measured values. Using these measured values, the tracking-type laser interferometer 40 can be used as a three-dimensional coordinate measurement device.
In addition, by using a plurality of tracking-type laser interferometers, using only the distance data (L) observed by the laser interferometer 72, trilateration for length can be performed and three-dimensional coordinate values can also be calculated.
However, in the tracking-type laser interferometer shown in FIG. 1, the position of the retroreflector 42 may become impossible to track when the laser light 74 is interrupted by an obstacle or the like between the tracking-type laser interferometer main body 50 and the retroreflector 42, or when a surface of the retroreflector 42 is dirty, or for some other reason. In such a case, labor is required in order to restart the measurement, such as a worker going to where the rotation mechanism control device 82 is located and operating the tracking-type laser interferometer main body 50 to fire the laser light 74 at the retroreflector 42, or moving the displacement body 22 (including the retroreflector 42) to the laser light 74 being emitted. In addition, because measurement is stopped partway through, measurement must be conducted once again. Because human beings are involved in this work, environmental changes (such as in temperature) may result. This is not beneficial to highly precise measurements.
Applicant notes that Japanese Patent No. 5,244,339 suggests a fan-shaped laser light be emitted to locate a retroreflector, while Japanese Patent Laid-open Publication No. 2010-190634 suggests scanning a spiraling laser light to locate a retroreflector. However, both of these require additional hardware.
The present invention has been devised in order to resolve the conventional concerns and, even when a laser light is blocked and tracking is no longer possible for a tracking-type laser interferometer using a laser interferometer to measure displacement, tracking can be automatically reset and measurement can be restarted with a simple method, and without additional hardware.